High density planar magnetic domain wall memory apparatus

ABSTRACT

A magnetic domain wall memory apparatus with write/read capability includes a plurality of coplanar shift register structures each comprising an elongated track formed from a ferromagnetic material having a plurality of magnetic domains therein, the shift register structures further having a plurality of discontinuities therein to facilitate domain wall location; a magnetic read element associated with each of the shift register structures; and a magnetic write element associated with each of the shift register structures, the magnetic write element further comprising a single write wire having a longitudinal axis substantially orthogonal to a longitudinal axis of each of the coplanar shift register structures.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/694,183, filed Mar. 30, 2007, the disclosure of which is incorporatedby reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to memory storage devices and,more particularly, to a high-density, planar magnetic domain wall memoryapparatus.

Dynamic Random Access Memory (DRAM) integrated circuit arrays have beenin existence for several years, with their dramatic increase in storagecapacity having been achieved through advances in semiconductorfabrication technology and circuit design technology. The considerableadvances in these two technologies have also resulted in higher andhigher levels of integration that permit dramatic reductions in memoryarray size and cost, as well as increased process yield.

A DRAM memory cell typically includes, as basic components, an accesstransistor (switch) and a capacitor for storing a binary data bit in theform of a charge. Typically, a first voltage is stored on the capacitorto represent a logic HIGH or binary “1” value (e.g., V_(DD)), while asecond voltage on the storage capacitor represents a logic LOW or binary“0” value (e.g., ground). A basic drawback of a DRAM device is that thecharge on the capacitor eventually leaks away and therefore provisionsmust be made to “refresh” the capacitor charge, otherwise the data bitstored by the memory cell is lost.

The memory cell of a conventional Static Random Access Memory (SRAM), onthe other hand, includes, as basic components, an access transistor ortransistors and a memory element in the form of two or more integratedcircuit devices interconnected to function as a bistable latch. Anexample of such a bistable latch is a pair of cross-coupled inverters.Bistable latches do not need to be “refreshed,” as in the case of DRAMmemory cells, and will reliably store a data bit indefinitely so long asthey continue to receive supply voltage. However, such a memory cellrequires a larger number of transistors and therefore a larger area ofsilicon real estate than a simple DRAM cell, and draws more power than aDRAM cell. Like a DRAM array, an SRAM array is also a form of volatilememory in that the data is lost once power is removed.

Accordingly, efforts continue to identify other types of memory elementsthat are capable of storing data states, that do not require extensiverefreshing, and that are non-volatile in nature. For example, certaintypes of magnetic memories have evolved that offer storage at anextremely low cost-per-bit, but generally suffer from performance thatis not competitive with semiconductor memories such as SRAM or DRAM.Presently, there is considerable effort in the field of magnetics tobring the large and slow (but inexpensive) magnetic memory technologieslike hard drives and the less commercially successful “bubble memory”devices into a higher performance realm, where it may replace SRAM orDRAM for certain applications. “Bubble memory” refers to the storage ofinformation in a linear series of “bubbles” of magnetization on a tapeof magnetic material. Through judicious application of magnetic fieldsto this fixed tape, the bubbles are made to move or shift along the tapeas in a shift register. By locating a read element at one position alongthe tape, it is possible to read out the state of the individual bits asthey are shifted along by the external magnetic field.

However, the initial concept of the bubble memory was slow tocommercialize for at least two reasons. First, it relied on the use ofexternal fields for shifting the magnetic bits, which is typically avery slow, “power hungry” process, and is more suited for operation on amacroscopic scale (e.g., the efficiency is greater if the entire planeshifts together, rather than shifting individual, small arrays of bits).Second, the macroscopic nature of conventional bubble memory impliesthat if there is a single defect in the “shift-register” track, then anexceedingly large number of bits will be rendered unusable. Moreover,redundancy and fusing schemes for yield improvement are therefore veryexpensive or impractical.

More recent developments in the field of spintronics have made a certaintype of microscopic memory possible, one having close similarities withrespect to the “macroscopic” bubble memory. This concept involves theuse of “domain walls” as the mechanism for storage of information, withsuch domain walls being located within microscopic (nanoscale) wires ofmagnetic material. The physics underlying the domain wall memory conceptare manifested through a local, microscopic means of shifting the bitsalong a shift register track. By flowing a sufficiently largespin-polarized current along the nanowire, enough force is imparted fromthe electrons onto the domain walls such that the domain walls may bemoved along the wire. In addition, certain techniques are used to pinthe domain walls at regular locations along the wire for simple,reliable readout of the information by a small number of read elementsfor many bits of information.

A key aspect for creating a practical, useful memory in this manner isthat the shift register tracks may be made quite small, and may beshifted locally, rather than with a global external magnetic field. Thisprovides a bridge between the speed of random access memory (single-bitstorage) and the high density (and low cost) of shift registers. Throughthe use of domain wall memory, a plurality of small shift registers maybe configured in an array fashion on a circuit. This provides thecapability of addressing and shifting each bit individually for maximumflexibility, while at the same time packing large densities of bitstorage into the miniscule nanowires. In addition, the shift registersmay be made small enough so that a production failure of a given shiftregister can be recovered through the use of additional redundant shiftregisters, thus eliminating the need for perfect yield of all devices ona given circuit.

In summary, conventional bubble memory suffers from limitations withrespect to speed, track density, and physical defects that are difficultto circumvent. Although the domain wall memory concepts described aboveoffer solutions to the problems of bubble memory, such newly proposeddomain wall memory concepts have, as a practical matter, been extremelycomplex and difficult to fabricate (e.g., 3-dimensional shift registerstructures). Accordingly, it would be desirable to be able to fabricatepractical domain wall memory structures in a more cost-effectiveproduction environment.

SUMMARY

The foregoing discussed drawbacks and deficiencies of the prior art areovercome or alleviated by a magnetic domain wall memory apparatus withwrite/read capability. In an exemplary embodiment, the device includes aplurality of coplanar shift register structures each comprising anelongated track formed from a ferromagnetic material having a pluralityof magnetic domains therein, the shift register structures furtherhaving a plurality of discontinuities therein to facilitate domain walllocation; a magnetic read element associated with each of the shiftregister structures; and a magnetic write element associated with eachof the shift register structures, the magnetic write element furthercomprising a single write wire having a longitudinal axis substantiallyorthogonal to a longitudinal axis of each of the coplanar shift registerstructures.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numberedalike in the several Figures:

FIGS. 1( a) and 1(b) are schematic top views of an existing, singlemagnetic domain wall shift register;

FIG. 2 is another top view of the shift register of FIGS. 1( a) and1(b), further illustrating write and read elements;

FIG. 3 is a schematic cross-sectional view of the shift register of FIG.2, depicting front-end CMOS control circuitry;

FIGS. 4( a) through 4(c) are a series of process flow steps illustratinga structure and method of forming write conductors for a high-density,planar magnetic domain wall memory device in accordance with anembodiment of the invention;

FIGS. 5( a) through 5(i) are a series of process flow steps illustratinga structure and method of forming a high-density, planar magnetic domainwall memory device in accordance with a further embodiment of theinvention;

FIG. 6 is a schematic top view of an exemplary high-density, planarmagnetic domain wall memory device comprising multiple, co-planar shiftregisters in accordance with a further embodiment of the invention.

FIG. 7 is a schematic top view of an exemplary high-density, planarmagnetic domain wall memory device having a compact write apparatus, inaccordance with still another embodiment of the invention; and

FIG. 8 schematically depicts an exemplary writing method of reducingfailures from electromigration in shift register domain wall memory, inaccordance with a further embodiment of the invention.

DETAILED DESCRIPTION

Disclosed herein is a high-density, planar magnetic domain-wall memorystructure having the additional advantages of speed and physicalerror-correction capability through, for example, redundancy and fusing.Briefly stated, multiple planar domain wall shift register tracks areformed through the use of existing semiconductor industry processingtechniques. By staggering multiple, in plane shift registers,accommodations are made for multiple, in plane read and write conductorsassociated with the individual registers. Moreover, since the planarstructure is concentrated in back-end-of-line (BEOL) structures that donot require extensive use of silicon transistors, one embodiment of theinvention utilizes the layering of multiple such in-plane structuresatop one other for extremely high-density memory arrays. Alternatively,the multiple, in plane shift registers may be aligned with one anotherso as to utilize a common write wire.

Referring initially to FIGS. 1( a) and 1(b), there is a schematic topview of a existing, single magnetic domain wall shift register structure100, illustrating the general principle of memory storage and shifting.The shift register structure 100 comprises a thin track 102 made of aferromagnetic material. The track 102 may be magnetized in small domainsor sections 104, in one direction or another, as indicated by thearrows. Bits are stored within the track 102 based on the presence orabsence of domain walls, which are located and detected at, for example,notches 106 in the thin magnetic track 102. However, othercharacteristics may also be used to define individual domain boundariessuch as, for example, physical overlapping of magnetic segments, varyinglayer thicknesses (e.g., by partially etching back or partially platingup every other domain), or using alternating types of magnetic materialsin the track 102. In other words, domain boundaries for storingindividual bits can be formed by physical discontinuities (e.g.,notches) or by material discontinuities.

Data within the register 100 is shifted through the application ofcurrent through a wire 108 connected at opposite ends of the track 102,as more particularly illustrated in FIG. 1( b). Depending upon theduration of the applied polarized electron current, a force is impartedthat is capable of shifting the domain walls from one notch to anadjacent notch. In the example depicted in FIG. 1( b), the direction ofthe applied current causes the data to shift one position to the right.Unless measures are taken to capture the data (the data at the rightmostdomain is shifted off the track 102), that bit will be lost.

FIG. 2 is another top view of the shift register 100 of FIGS. 1( a) and1(b), further illustrating write and read elements. In particular, awrite element positioned at one end of the shift register 100 includes aconductor or wire 110 having a constriction 112 (i.e., a narrow portion)formed therein corresponding to a domain 104 or a domain boundary(notches 106). Although FIG. 2 shows the write wire 110 positionedbeneath a domain boundary, it will be noted that the wire may also bepositioned beneath a domain instead. The write element wire 110 carriesa current orthogonal to the magnetic memory element, with the resultingmagnetic field being magnified at the constriction 112 in order tofacilitate writing of the domain wall.

In addition, a read element 114 is positioned at the opposite side ofthe shift register 102 with respect to the write element. In the exampleillustrated, the read element 114 is embodied by a magnetic tunneljunction (MTJ). As indicated above, in order to maintain data in theshift register 100, a closed-loop shift register may be created byfeeding back “read” data to the write element as the data in the shiftregister 102 is shifted by the application of current through wire 108.A read wire 116 is also coupled to the MTJ 114.

FIG. 3 is a schematic cross-sectional view of the shift register 100 ofFIG. 2, particularly illustrating connections to the front-end CMOSshift, read and write control circuitry. Because at most threetransistors are needed for the entire shift register, the memory isheavily BEOL-loaded, and stacking of multiple structures could beemployed to density the memory without using up all the silicon realestate beneath. However, in terms of a single plane, a problem existswith regard to forming multiple, co-planar shift registers as a resultof the use of separate read and write wires for each.

Accordingly, FIGS. 4( a) through 4(c) are a series of process flow stepsillustrating a structure and method of forming write conductors for ahigh-density, planar magnetic domain wall memory device in accordancewith an embodiment of the invention. As write wire efficiency isimproved by proximity to the magnetic shift register element, thedepicted embodiment is ideally suited for using a well-controlled, thindielectric cap atop the write wires to accurately (and closely) spacethe ensuing magnetic film from the write wire without short circuiting.

As particularly shown in FIG. 4( a), a plurality of write wires 402 areformed in damascene fashion within an interlevel dielectric layer 404above the silicon CMOS level 406 of a semiconductor device. Vias, suchas via 408, are used to connect the write wires 402 to the associatedswitching transistors located on the silicon CMOS level 406. As shown inthe top view of FIG. 4( b), the damascene write wire trenches arepatterned with constrictions 410 at locations corresponding to the shiftregister in order to assist in magnetic field enhancement, therebyenabling domain wall formation in the register. Moreover, it will benoted that the constrictions 410 are staggered along a longitudinaldirection of the write wires 402, with respect to one another, so as toallow multiple shift registers to be formed in the same horizontalwiring level.

In FIG. 4( c), a thin dielectric cap layer 412 is shown formed atop thewrite wires 402 and interlevel dielectric layer 404. The cap layer 412forms a thin insulating barrier having good across-wafer uniformity, andat a well-known thickness. Through accurate control of the thickness ofthis film 412, the write wires may be positioned very closely to themagnetic film to be deposited above the cap layer 412, without danger ofshort circuits. Such close positioning will reduce the necessary currentin the write wire needed to switch the magnetization state of the domainatop the write wire.

Referring now to FIGS. 5( a) through 5(i), there are shown are a seriesof process flow steps illustrating a structure and method of forming ahigh-density, planar magnetic domain wall memory device in accordancewith a further embodiment of the invention. More specifically, FIGS. 5(a) through 5(i) illustrate the formation of the shift register element,the read element, and the wiring connections to the shift registerelement.

As shown in FIG. 5( a), a blanket stack of films are deposited atop thewrite wire/dielectric layer structure shown in FIG. 4( c). For purposesof clarity, the write wire/dielectric layer structure is notspecifically illustrated in the FIG. 5 sequence. In the embodimentillustrated, the films correspond to materials used in a magnetic tunneljunction, although it will be appreciated that other layers may be usedwhere a different read device is employed. For an MTJ device, the filmsinclude a free layer 502, a tunnel barrier 504 over the free layer 502,a pinned layer 506 over the tunnel barrier 504, and a cap layer 508.Specific materials used for the MTJ device layers may be in accordancewith those known in the art.

In FIG. 5( b), the cap and pinned layers 508, 506, are lithographicallypatterned and then etched to define an MTJ element, corresponding to alocation near an end of the associated shift register. It will be notedthat the tunnel barrier layer 504 and free layer 502 need not be etchedfor MTJ device formation. Then, in FIG. 5( c), an encapsulating layer510 is formed over the device, followed by another lithographicpatterning process to define the shift register, characterized by anelongated, track shape with discontinuities (e.g., notches 512) formedtherein for domain wall location. The domain wall locatingdiscontinuities may be created with the same photomask that defines theelongated track shape, or the discontinuities may alternatively beformed at an earlier stage using a technique other than notches. A topview of the shift register structure 514 formed in FIG. 5( c) isillustrated in FIG. 5( d), which better illustrates the shape of theshift register with notch discontinuities 512 and MTJ read element 516.

Referring next to FIG. 5( e), an interlevel dielectric layer 518 isformed and planarized over the structure of FIG. 5( d), in preparationof contact formation to the ends of the shift register 514 and MTJ readelement 516. In FIG. 5( f), vias 520 are opened at opposite ends of theshift register, stopping on the free layer 502. Another via 521 isformed so as to stop on the cap layer 508 of the MTJ element. A top viewtaken along the arrows in FIG. 5( f) is shown in FIG. 5( g).

Proceeding to FIG. 5( h), a trench etch is performed in accordance withdual damascene processing techniques, followed by conductive metal fillso as to form shift current wires 522 and read wire 524. FIG. 5( i) is atop view along the arrows of FIG. 5( h). Again, it will be noted thatthe write wires, formed below the free layer 502 are not illustrated inthe FIG. 5 sequence.

FIG. 6 is a top-down view illustrating the staggering of severalco-planar shift registers 514 for dense packing of memory elements on aplanar surface with individual write wires assigned to each shiftregister element. As can be seen, the write wires 402 (withconstrictions 410) are disposed at one end of the shift registers 514,while the MTJ elements 516 and associated read wires 524 are disposed atthe other end of the registers 514. It should be appreciated thatalthough the write wires 402 are shown as being formed below the shiftregisters 514 and the MTJ read elements 516 as being formed above theshift registers, other arrangements are also contemplated.

For example, the MTJ read elements 516 might be formed below the shiftregister 514, or even adjacent to (i.e., in the same plane as) the shiftregister 514. Likewise, the location of the write wire 402 andconstrictions 410 may be above the shift register 514, or disposedvertically with respect to the shift register 514. In other words, thewrite wire can be formed as a via which carries current vertically withrespect to the wafer substrate.

In lieu of an MTJ read element 516, other read mechanisms, such as GMR(giant magnetoresistance) sensors may also be employed. Still othercontemplated variations include, but are not limited to: enhanced writewire configurations, such as high-permeability field-focusing elements(also called ferromagnetic field concentrators), and nonlinear shiftregisters, such as those including a curve, bend or other nonlinearshape within a circuit plane.

Referring now to FIG. 7, there is shown a top-down view illustrating analternative embodiment of several co-planar shift registers 514 fordense packing of memory elements. In the embodiment illustrated, theneed for staggering of the shift register elements 516 is eliminated. Inone implementation of the structure shown in FIG. 7, a single commonwrite wire 402 is associated with several shift register elements 516.Depending on the current drive capability of the circuit, a constriction(e.g., element 410 of FIG. 6) could be used beneath each shift registerelement, or (as specifically shown in FIG. 7) a simple straight writewire 402 could be employed. In either instance, the alignment of themultiple co-planar shift registers 514 results in the modifiedconfiguration of the read wires 524 as shown in FIG. 7. For example, therightmost read wire corresponding to the bottom shift register 514 issubstantially straight while the read wires corresponding tosuccessively higher shift registers become more L-shaped. Other readwire configurations, however, are also contemplated.

With regard to writing a desired bit to a selected shift register(s) 514using the single write wire configuration of FIG. 7, a confluence of twomechanisms is utilized: (1) a write current (along wire 402) of desireddirectionality used to define the direction of the bit's magnetization,and (2) a shift current (represented by arrow 602) applied only alongthe desired shift register(s), to “enter” the bit into position on theshift register's leftmost active storage cell 604. Write currents alongwrite wire 402 without an associated shift current 602 would not resultin the switching the state of cell 604, and thus will not affect thestorage state of the shift registers. The element to the left of cell604 is intended as a dummy (non-storage) element to facilitate reliablewriting by spacing the shift register end a desired distance away fromthe edge of the write wire 402.

It is known in the art that a relatively large current density isrequired for shift currents 602 to effectively shift the domain wallsalong the shift register element. Unipolar operation, in which bits arealways shifted in the same direction, is favored for simplicity andpacking density. However, in combination with high current densityshifting, such operation can lead to device failure over time throughelectromigration. Accordingly, FIG. 8 depicts an exemplary scheme forreducing device failure due to electromigration by using non-shiftingregisters as return current paths for the shift current in activelyshifting registers. Because shifting of the domain walls requirescurrent above a certain threshold, current below that threshold levelmay be passed through a shift register without shifting the domainwalls. Accordingly, by splitting the return path of a given register'sshift current 602 (e.g., through wire 522 b) into multiple registercurrents 702 (e.g., through wires 522 a, 522 c, 522 d), the registerwith supplied with current 602 is shifted while, at the same time, notshifting any registers with reduced current 702. The use of the returncurrent in this manner will counteract electromigration for increaseddevice lifetime.

While the invention has been described with reference to a preferredembodiment or embodiments, it will be understood by those skilled in theart that various changes may be made and equivalents may be substitutedfor elements thereof without departing from the scope of the invention.In addition, many modifications may be made to adapt a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe appended claims.

1. A magnetic domain wall memory apparatus with write/read capability,comprising: a plurality of coplanar shift register structures eachcomprising an elongated track formed from a ferromagnetic materialhaving a plurality of magnetic domains therein, the shift registerstructures further having a plurality of discontinuities therein tofacilitate domain wall location; a magnetic read element associated witheach of the shift register structures; and a magnetic write elementassociated with each of the shift register structures, the magneticwrite element further comprising a single write wire having alongitudinal axis substantially orthogonal to a longitudinal axis ofeach of the coplanar shift register structures.
 2. The apparatus ofclaim 1, wherein ends of each shift register structure are aligned withone another.
 3. The apparatus of claim 2, wherein the write wireincludes a plurality of constrictions therein, the constrictions locatedat a point corresponding to the location of one of the plurality ofdiscontinuities in the each of the associated shift register structures.4. The apparatus of claim 1, wherein: the write wire is locatedproximate a first end of the shift register structures, and each readelement is located proximate a second end thereof; and an endmost of theread elements is substantially straight and the remaining read elementsare substantially L-shaped.
 5. The apparatus of claim 1, furthercomprising: a plurality of shift current wires associated with each ofthe shift register structures; wherein the plurality of shift currentwires are configured such that the shift current wires corresponding tounselected shift registers act as a current return path for shiftcurrent passing through a selected shift register, the current returnpath being in an opposite direction with respect to the direction of theshift current.
 6. The apparatus of claim 1, wherein each read elementcomprises a magnetic tunnel junction (MTJ).
 7. The apparatus of claim 1,wherein the plurality of discontinuities in each track are defined by aplurality of vertical notches formed within the ferromagnetic material.8. The apparatus of claim 1, wherein the plurality of discontinuities ineach track are defined by physical overlapping of magnetic segments. 9.The apparatus of claim 1, wherein the plurality of discontinuities ineach track are defined by using alternating types of magnetic materialsin the track.
 10. The apparatus of claim 1, wherein the plurality ofdiscontinuities in each track are defined by different materialthicknesses for adjacent domains.